people act like using the same (inhouse) engine for your game series, is bad when like:
making a new one from scratch is a lot of wasted effort (especially if you have custom tools made)
game engines can be updated and new features added/bugs patched
things like physics, textures, UI, quest management, and basically everything exposed to the player can be changed each game
Fallout 4 and Morrowind "technically" use the same engine but they're >10 years apart and a lot changed in that time.
If you dumped Fallout 4's map, characters, quests and items into the Morrowind engine I doubt it'd run tbh, but if it did you wouldn't have things like face animations, idle/sandbox animations, much of the physics, or any of the settlement building. those didn't exist in 2002's version of the engine.
the point of keeping and developing the same engine is it works with their dev tools. it uses the same master files + patch files system. it tracks stats the same way internally. it allows for items to be created in a similar way, for item effects to work in the same way (btw the way effects and spells work is messy as all hell but that's part of the fun). it allows for easier debugging and testing.
also, if Bethesda ever ditch Creation Engine (or radically redesign it for version 3 in the future), it'll be the death of modding on their games.